This past summer I experienced a situation with my Bounder, built on a Ford F53 chassis, that gave me cause for concern and for a while, truly stumped me. I had hooked up our tow car around dusk one night and checked the operation of both the taillights and the four way flashers (which indicated that both the brake and signals would operate correctly when called upon to do their task). I drove a short distance and when I signaled my right turn -- no signal. I thought a bulb might have blown, although I knew that usually the remaining bulbs would light but not flash. I tried the other signal as I merged onto the highway, -- no signal. I must have a defective flasher, I thought. On the Ford chassis there is only one flasher. A few minutes later as I contemplated the problem I decided to try the four-way flasher, which should determine if the flasher was the problem. Well lo and behold all flashers worked fine. A few minutes later I changed lanes and the signal failed to operate. This continued until close to my destination when the signals once again began operating as designed. I checked the tow car and Bounder signals when I stopped and everything was fine. I disconnected and thought little more about it until the next day when I moved the Bounder without the tow car attached and once again, I had no signals (or four-way flashers). I concluded that I had a defective flasher. Since it was the weekend and I was dressed for a wedding I was attending, I decided between the ceremony and the reception to go to a large auto-parts store and ask for a replacement flasher. I did not take the original with me. Their cross-reference chart showed only the F350 (a large pickup truck) as using a 3-prong flasher and that resonated with me on those that I had looked at in the past, so I purchased the flasher.
After the reception I carefully drove to our overnight campground, whereupon the next day I exchanged the flasher. As soon as I installed it and operated the signals I blew the brake light fuse. Replacing it and trying again resulted in the same problem. I concluded that I had a wiring problem. I suspected the tow car harness and I proceeded to check for power on the appropriate leads. All was well other than some corrosion, which I attended to with di-electric grease. The brake lights on both sides worked and all wiring was fine. As long as I did not use the flashers everything worked. I retraced the events in my mind and thought the problem must be the flasher and I re-installed the original and gave the mechanic's favorite - "tap" and all worked properly. I decided that I should obtain the original Ford Flasher and with it installed the signals should be back to proper operation. I installed the Ford "signal relay" and my signals still did not work. I had asked the service shop for a copy of the wiring diagram for the switch and had talked to the service advisor/mechanic at the Ford dealer where I usually have my Bounder serviced. He suggested that the problem is rarely the multi-function switch although I should check it carefully but in some instances replacing the Three Prong relay with a Heavy Duty Two Prong relay inserted in such a way that the signals operate, would probably cure the problem. He was not sure why that was, but his experience was that it is an effective solution.
The circuit diagrams showed me that the wiring from each corner of the motorhome (all four individual signal lights) are routed to the signal switch and that the four-way flasher and signal switch are combined into one unit in the multi-function switch. The brake light connection is actually made by joining the two rear signal lights together and feeding the power from the cold side of the brake switch to the signal switch, which passes the power through to the brake lights. Once again I traced the wiring to each corner and all was fine. The wire from the Flasher Relay to the switch was OK as well. In addition there was power, both for the Four-way function (direct from the battery) and through the ignition switch for the regular signals.
So why did the Ford Relay not operate? I am not certain, but I did determine that the third wire at the relay was a ground connection and that the conventional Three Prong flasher would connect the power and ground together, obviously causing the power supply fuse to blow. The Two Prong Flasher operates on a different principal. It depends on a certain load to open a bi-metallic switch, which lights the appropriate signal bulbs, then opens the circuit causing the signal to flash on and off depending, on which direction the signal lever is set. It does not use the ground and it depends to a certain extend on the load (number of bulbs) to operate. A Two Prong Heavy Duty Flasher was inserted and all signal and Four-way flasher operation resumed as normal.
I was curious about the answer to this problem, so I posted the question on the Bounder E-group/Yahoo group. Mr. Jim Pourroy, the Technical Service Trainer Western Region for Fleetwood replied
It is acceptable to use a Wagner 552 flasher in place of the Ford electronic flasher, and you are correct in the importance of connecting it correctly. It needs to be plugged into the terminals that have the "blue and orange" wires AND NOT the black (ground) wire. The Ford flasher uses an IC (Integrated Circuit) timing circuit to pulse the relay and thereby cause the lights to flash. That is why the ground wire is needed.
So now you know. No one can explain why a new relay failed to work any better than my old one. What is important is that you must replace the Flasher with a TWO PRONG Flasher if buying an aftermarket flasher. Most people would pull the wire off the flasher and go to the auto-parts store replacing the Three Prong Flasher with a similar new one. Remember all Ford chassis have a single Flasher Relay (capable of flashing up to 20 bulbs) located at the base of the steering column on the "support panel.
As a note for the Chevrolet P30 and Workhorse Chassis owners, your rigs have two flashers: one usually connected in the wiring harness along side of the steering column for the regular signal, and a second flasher in the fuse panel for the four-way system. On some of the newer Bounders on these chassis, the two flashers are in the fuse panel under the dash.
As far as I know the diesel chassis have two flashers located in similar locations to the Chevrolet Chassis.